This invention relates to a method for producing a manganese oxide octahedral molecular sieve (OMS). More particularly, this invention relates to a method for producing a manganese oxide octahedral molecular sieve which is carried out in an open system, e.g., under refluxing conditions.
Manganese oxide octahedral molecular sieves (OMS) possessing mono-directional tunnel structures constitute a family of molecular sieves wherein chains of MnO.sub.6 octahedra share edges to form tunnel structures of varying sizes. Such materials have been detected in samples of terrestrial origin and are also found in manganese nodules recovered from the ocean floor. Manganese nodules have been described as useful catalysts in the oxidation of carbon monoxide, methane and butane (U.S. Pat. No. 3,214,236), the reduction of nitric oxide with ammonia (Atmospheric Environment, Vol. 6, p.309 (1972)) and the demetallation of topped crude in the presence of hydrogen (Ind. Eng. Chem. Proc. Dev., Vol. 13, p.315 (1974)).
Pyrolusite, .beta.-MnO.sub.2, is a naturally occurring manganese oxide characterized by single chains of MnO.sub.6 octahedra which share edges to form (1.times.1) tunnel structures which are about 2.3 .ANG. square. Ramsdellite, MnO.sub.2, is a naturally-occurring manganese oxide characterized by single and double chains of MnO.sub.6 octahedra which share edges to form (2.times.1) tunnel structures which are about 4.6 .ANG. by about 2.3 .ANG. square. Nsutite, .gamma.-MnO.sub.2, is a naturally-occurring manganese oxide characterized by an intergrowth of pyrolusite-like and ramsdellite-like tunnel structures. Pyrolusite, ramsdellite and nsutite do not possess cations in their tunnel structures.
The hollandites are naturally occurring hydrous manganese oxides with tunnel structures (also described as "framework hydrates") in which Mn can be present as Mn+.sup.4 and other oxidation states, the tunnels can vary in size and configuration and various mono- or divalent cations can be present in the tunnels. The hollandite structure consists of double chains of MnO.sub.6 octahedra which share edges to form (2.times.2) tunnel structures. The average size of these tunnels is about 4.6 .ANG. square. Ba, K, Na and Pb ions are present in the tunnels and coordinated to the oxygens of the double chains. The identity of the tunnel cations determines the mineral species. Specific hollandite species include hollandite (BaMn.sub.8 O.sub.16), cryptomelane (KMn.sub.8 O.sub.16), manjiroite (NaMn.sub.8 O.sub.16) and coronadite (PbMn.sub.8 O.sub.16).
The hydrothermal method of synthesizing a manganese oxide octahedral molecular sieve possessing (2.times.2) tunnel structures such as those possessed by the naturally-occurring hollandites is described in "Hydrothermal Synthesis of Manganese Oxides with Tunnel Structures," in Synthesis of Microporous Materials, Vol. II, 333, M. L. Occelli, H. E. Robson Eds. Van Nostrand Reinhold, N.Y., 1992 and R. Giovanili and B. Balmer, Chimia, 35 (1981) 53. Such synthetic octahedral molecular sieves having (2.times.2) tunnel structures are referred to in the art by the designation OMS-2. The (2.times.2) tunnel structure of OMS-2 is diagrammatically depicted in FIG. 1.
The hydrothermal method of synthesizing OMS-2 involves autoclaving an aqueous solution of manganese cation and permanganate anion under acidic conditions, i.e., pH&lt;3, at temperatures ranging from about 80.degree. to about 140.degree. C. in the presence of counter cations having ionic diameters of between about 2.3 and about 4.6 .ANG.. The counter cations can serve as templates for the formation of OMS-2 product and be retained in the tunnel structures thereof. Based on analytical tests, OMS-2 produced via this method is thermally stable up to about 600.degree. C.
Alternatively, OMS-2 can be produced by the method disclosed in R. Giovanili and B. Balmer, Chimia, 35 (1981) 53. Thus, when manganese cation and permanganate anion are reacted under basic conditions, i.e., pH&gt;12, a layered manganese oxide precursor is produced. This precursor is ion exchanged and then calcined at high temperatures, i.e., temperatures generally exceeding about 600.degree. C., to form OMS-2 product. Analytical tests indicate that OMS-2 produced via this method is thermally stable up to about 800.degree. C. and the average oxidation state of manganese ion is lower.